Definetly allen head...... you may need to invest in a new plug, and make sure you have quality allen keys..... i bought a set that was 99.5% good fit, and they slowly did damage to thingsIs it?! I thought it was but it just didn't feel right & it was starting to slip so I stopped. Maybe do the blue towel thing & tap it with a mallet a few times? It was certainly in there more than 8 ft lbs that's for damn sure?!!
over time, and then i got a set that where such a good fit yu had to make sure they were lined up perfect or they didn't like to go in......needless to say, anything they remove and install is
As far as the o ring goes, you can carefully remove it and reverse it to make sure it seals....done that before.....i also gently boil orings in water for 10 mins which can help swell them.